1. Field of the Invention
This invention relates to measuring gamma radiation emitted from a subsurface formation. In particular, the measuring is performed within a borehole.
2. Description of the Related Art
A variety of geologic formations contain reservoirs of oil and gas. Measuring properties of the geologic formations provides information that can be useful for locating the reservoirs of oil and gas. Typically, the oil and gas are accessed by drilling boreholes into the subsurface of the earth. The boreholes also provide access to take measurements of the geologic formations.
Well logging is a technique used to take measurements of properties of the geologic formations from the boreholes. In one embodiment, a logging instrument is disposed in a drill string in proximity to a drill bit. The logging instrument is used to take the measurements and send data via telemetry to the surface for recording. This type of well logging is referred to as “logging while drilling” (LWD). One type of LWD measurement involves measuring naturally occurring gamma radiation (or gamma rays) from the geologic formations.
The geologic formations may include regular features (bedding planes and formation contacts) and irregular features (faults, nodules and changes in cementation). In a quest for oil and gas, it is important to know about the location and composition of these regular and irregular features. In particular, it is important to know about the bedding planes with a high degree of accuracy so that drilling resources are not wasted.
Measuring naturally occurring gamma radiation is one way to determine characteristics of the bedding planes. For example, a gamma radiation detector may be used as a component of the logging instrument to measure the naturally occurring radiation in the borehole. In some embodiments, scintillator materials are used for gamma radiation detection.
Radiation detectors using scintillation materials are usually optically coupled to a devise such as photomultiplier tube (PMT). Interaction of radiation within the scintillator causes emission of at least a photon by the scintillator. Subsequently, the photon is detected in the PMT and accounted for by appropriate electronics. Gamma rays may enter the gamma radiation detector from any angle. As long as a gamma ray interacts within the gamma radiation detector, the gamma radiation detector will output an electrical signal regardless of the angle of entry. As the gamma radiation detector is moved along the borehole, gamma rays emitted from a rock formation (a bedding plane itself cannot emit gamma radiation, but the contrast between two formations or features is detectable by measuring gamma radiation) may enter the gamma radiation detector and be detected.
Various shapes and forms of gamma radiation detectors may provide various types of information about the bedding planes. A cylindrically shaped gamma radiation detector of a certain length provides opportunities for gamma rays to interact along the length as the gamma radiation detector moves by the formation bedding planes. On the other hand, if the gamma radiation detector was, hypothetically, only a point, there would be fewer opportunities for gamma rays to interact as the gamma radiation detector moves by the formation bedding planes. A gamma radiation detector that has a geometry that is equivalent to a point can detect changes in radiation as the detector is moved through the borehole with better spatial resolution than the cylindrically shaped gamma radiation detector. Cylindrically shaped gamma radiation detectors will have less spatial resolution and therefore will result in less accurate knowledge of the formation bedding planes.
What are needed are techniques for making measurements of gamma radiation from a subsurface formation where the techniques provide improved spatial resolution along an axis of the borehole.